"In a paper published in 2008 and focused on the then latest version (v5.1a) and its plausible deniability, a team of security researchers led by Bruce Schneier states that Windows Vista, Microsoft Word, Google Desktop, and others store information on unencrypted disks, which might compromise TrueCrypt's plausible deniability. The study suggested the addition of a hidden operating system functionality; this feature was added in TrueCrypt 6.0. When a hidden operating system is running, TrueCrypt also makes local unencrypted filesystems and non-hidden TrueCrypt volumes read-only to prevent data leaks. The security of TrueCrypt's implementation of this feature was not evaluated because the first version of TrueCrypt with this option had only recently been released."
It clearly states that it did not evaluate the security plausible deniability of the 'hidden OS' feature.
This is basically saying that if you mount a hidden partition, you may leak information via things like browser cache that ends up getting saved to unencrypted areas. On the other hand, this says nothing about the case where you have the full drive encrypted and boot a different OS if you mount the hidden partition via the bootloader.
Citation?
>> I'm not sure if they fixed it since then.
Maybe you could find out and post something useful.